Recently, with the development of mobile applications, high-capacity energy source is required. The lithium secondary battery is a representative example. In the currently used lithium secondary battery, carbonaceous materials are used as anode active material. The theoretical capacity of the carbonaceous material is 372 mAh/g. Since the capacity of the presently commercially available batteries is around 360 mAh/g, it has almost reached the theoretical capacity limit. Accordingly, for preparation of high-capacity batteries, development of a new material capable of replacing the carbonaceous anode active material is necessary.
Representative materials as the new high-capacity anode active material include metallic materials such as silicon (Si), tin (Sn), etc. that allow intercalation/deintercalation of lithium (Li) via alloying reactions with lithium. However, when a metal such as silicon (Si), tin (Sn), etc. alone is used as the anode active material, volume change during charging-discharging is very large, as much as 300-400%, resulting in separation from the electrode and significant deterioration of cycle characteristics. Therefore, it is hard to be actually applied in batteries.
To overcome this problem, alloys of silicon (Si) or tin (Sn) with other metals (i.e., Si-M alloys and Sn-M alloys, M is a metal element) have been studied by many researchers. These alloy-type materials are known to form a single-element phase of silicon (Si), tin (Sn), etc. that can bind with lithium, a metal silicide (MxSiy) phase that does not bind with lithium, a metal-tin (Sn) alloy (MxSny) phase, or the like. The single phase of silicon (Si), tin (Sn), etc. is capable of binding with and release from lithium during charging-discharging and, thus, provides battery capacity through electrochemical reactions. The metal silicide (MxSiy) phase and the metal-tin (Sn) alloy (MxSny) phase do not bind with lithium but are known to suppress volume expansion of the anode active material by suppressing the volume change of the single phase of silicon (Si), tin (Sn), etc.
In this regard, in order to control electric capacity and cycle characteristics, the phases of the anode active material have been divided into a phase mainly essentially of Si and a phase mainly consisting of Si-metal alloy, and the ratio of the phases (Japanese Patent Laid-Open No. 2006-164960), or the particle size of the phases (Japanese Patent No. 4344121, Korean Patent No. 911799) have been controlled.
However, in spite of these efforts, the electric capacity and cycle characteristics are not satisfactorily controlled. Accordingly, there is a need of an anode active material allowing a better control of these properties.